linux-kernel - Re: [PATCHv6 1/2] i2c: designware: introduce a custom scl recovery for SoCFPGA platforms

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Message-ID: <YrI6EeVkkWVMNPFY@shikoro>
Date:   Tue, 21 Jun 2022 23:37:21 +0200
From:   Wolfram Sang <wsa@...nel.org>
To:     Dinh Nguyen <dinguyen@...nel.org>
Cc:     jarkko.nikula@...ux.intel.com, andriy.shevchenko@...ux.intel.com,
        mika.westerberg@...ux.intel.com, robh+dt@...nel.org,
        krzk+dt@...nel.org, linux-i2c@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCHv6 1/2] i2c: designware: introduce a custom scl recovery
 for SoCFPGA platforms

On Mon, Jun 20, 2022 at 06:01:08PM -0500, Dinh Nguyen wrote:
> The I2C pins on the SoCFPGA platforms do not go through a GPIO module,
> thus cannot be recovered by the default method of by doing a GPIO access.
> Only a reset of the I2C IP block can a recovery be successful, so this
> change effectively resets the I2C controller, NOT any attached clients.

I am afraid here is a serious misunderstanding. The I2C bus recovery
procedure is a documented mechanism how to get a stalled bus back in the
case that a client device holds SDA low. This mechanism consists of 9
SCL pulses. A reset of the IP core is *not a recovery*. If SocFPGA
cannot togle SCL in some way, it cannot do recovery and
adap->bus_recovery_info should be NULL. Or did I miss something?

> +static int i2c_socfpga_scl_recovery(struct i2c_adapter *adap)
> +{
> +	struct i2c_bus_recovery_info *bri = adap->bus_recovery_info;
> +
> +	bri->prepare_recovery(adap);
> +	bri->unprepare_recovery(adap);
> +
> +	return 0;
> +}

See, this function is named scl_recovery, but there is no SCL involved.
This is why I think there is the misunderstanding here.

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